[A-AS11] Atmospheric Chemistry

[AAS11-04] Hindcasting simulation of global methane concentration with
a Chemistry Climate Model

*Tatsuya Nagashima¹, Minoru Chikira¹, Kohei Ikeda¹, Tomoo Ogura¹, Hiroshi Tanimoto¹, Takashi Sekiya², Kengo Sudo³ (1.NIES National Institute of Environmental Studies, 2.JAMSTEC Japan Agency for Marine-Earth Science and Technology, 3.Nagoya University)

Keywords:Methane, Chemistry Climate Model, Hindcasting simulation, Climate change

Recent significant temperatures rise has raised concerns about the achievement of the 1.5°C target of the Paris Agreement, and efforts to reduce emissions of Short-Lived Climate Forcers (SLCFs) in addition to reducing carbon dioxide (CO2) emissions have become even more important. There are high expectations for reducing emissions of methane (CH4), which has the second largest radiative forcing after CO2, to mitigate climate change. In addition to emissions, atmospheric transport and chemical reactions are important in determining CH4 concentrations in the atmosphere. Therefore, even if CH4 emissions are reduced, it is not easy to quantitatively evaluate to what extent CH4 concentrations will decrease and to what extent temperature rise will be mitigated. Chemistry-Climate Model (CCM), which is a numerical model that explicitly represents these complexly related atmospheric emissions, transport, and chemical processes, is an essential tool for scientifically approaching these important issues. However, in most of the numerical model studies to evaluate the climate impact of CH4 changes, calculations have been performed by specifying the CH4 concentration in the model (concentration-driven experiments), and only a limited number of studies have been performed to directly evaluate the effect of emission reductions on the climate by inputting CH4 emissions to CCM and calculating CH4 concentrations in the model (emission-driven calculations). We are working on directly evaluating the climate impact of CH4 emission changes by performing emission-driven simulations using the MIROC6-CHASER model, a global-scale CCM. Here, we report the results of simulations to reproduce past CH4 concentrations driven by CH4 emission changes from pre-industrial (PI) era (1850) to the present (2014) using the MIROC6-CHASER model as an atmospheric model setup (exogenously providing Sea Surface Temperature (SST) and sea ice data). This model can calculate the atmospheric concentrations of Ox, NOx, HOx, VOCs including CH4, halogenated compounds, and atmospheric aerosols, and can explicitly calculate changes in meteorological fields through the modulation of atmospheric radiation due to changes in the concentrations of these chemical species. The horizontal resolution was T42 (grid spacing was approximately 320 km), and 36 layers were arranged vertically from the surface to approximately 55 km. We first evaluated the emissions that would reproduce the estimated global surface CH4 concentration in the PI era of ~800 ppbv, and obtained a value of approximately 163 TgCH4/yr. Then, we fed the model historical changes in SST and sea ice, increased anthropogenic CH4 emissions from fossil fuels, agriculture, waste management and so on toward the present, and calculated the changes in atmospheric CH4 concentration. The model was able to reproduce the observed increase in concentration well. However, after the 1970s, there has been a tendency to underestimate the observed concentrations (up to about 100 ppbv). This result is consistent with the results of previous CCM calculations, although the cause is not yet well understood. From the observation, it is known that there are large interannual variations in the rate of change in CH4 concentration, and it is known that the increasing rate in CH4 concentration slowed down toward the early 2000s before increasing again, and the MIROC6-CHASER model was able to capture this characteristic well. In a sensitivity simulation in which the SST and sea ice were kept in the PI state and no climate change such as global warming occurred, the surface concentration of CH4 increased by up to 4% compared to when climate change was applied. Conversely, this suggests that past climate change had the effect of reducing CH4 concentrations. In the presentation, the results of an analysis of the mechanism behind this will be reported.